1. Field of the Invention
The present invention relates to a plasma processing apparatus utilizing plasma generated by microwaves.
2. Description of the Related Art
In recent years, ICs (integrated circuits) have become smaller and smaller and the diameter of wafers have become larger and larger, which requires formation of uniform thin films having a large diameter. In particular, gate oxide films should be formed so as to be thin and uniform, because they affect the characteristics of ICs. Therefore, thin films such as gate oxide films are formed with a plasma processing apparatus using microwaves (e.g., 2.45 GHz). In a plasma processing apparatus that uses microwaves, plasma having a high density and a low electron temperature can be obtained with high frequency microwaves. This allows the effects of thin films such as gate oxide films on electrical breakdown or physical breakdown to be suppressed. Thus, when microwaves are used, thin films causing little damage can be formed efficiently. However, the wavelength of the microwaves is about the same size as the wafer diameter, so that when forming uniform thin films having a large diameter using microwaves, the films are susceptible to the properties of the waves (as described below).
Furthermore, in order to uniformly form thin films having a large diameter, a method is used that utilizes the ability of microwaves to be reflected and absorbed by plasma in order to makes the plasma reflect and absorb non-uniform microwaves. In this method, for example, a uniform thin film is formed by introducing microwaves only from the outside of a round or cylindrical processing chamber to introduce non-uniform microwaves, and plasma is allowed to absorb the non-uniform microwaves in order to obtain balance. In addition, another method is disclosed in which an introduction window from which microwaves are introduce is branched, and microwaves are introduced uniformly to a dielectric in a processing apparatus in order to form thin films having a large diameter (e.g., see JP 8-316198A).
Furthermore, in order to form uniform thin films having a large diameter, a slot plate for uniformly introducing microwaves to a dielectric is used in such a plasma apparatus. By using the slot plate, the microwaves become uniform, and the plasma generated by the microwaves becomes uniform. This uniform plasma makes it possible for the gas to be separated and excited uniformly so that a uniform thin film can be formed. For the slot plate, a thin metal plate is used in order to prevent protrusions that may be produced when forming a rectangular slot that serves as an introduction window for microwaves, prevent disturbances in the electric field strength distribution of the microwaves due to the thickness of the slot plate, and the like. In situations in which a thin metal plate is used, when the degree of coupling of the microwaves introduced from the thin metal plate is too large, electric sparks, anomalous discharges, or other problems will be produced. Therefore, the length of the long sides of the slots (hereinafter referred to as slot length) of the slot plate is set to be sufficiently shorter than half of the wavelength of the microwaves above the slot plate.
In situations in which microwaves are introduced to a dielectric from the slots of an antenna, a technique for reducing the thickness of the antenna is disclosed in order to increase the electric field strength of the microwaves introduced to a dielectric from slots having a slot length sufficiently shorter than a half of the wavelength of the microwaves propagating in the antenna (e.g., see JP 2002-50615A).
In the plasma processing apparatus, a dielectric or the like that is a region in which microwaves for generating plasma propagation is formed in a cylindrical or a round shape that matches the shape of a sample (e.g., silicon wafer). For this reason, as shown in FIG. 46, the microwaves are reflected from a wall surface 14 in the cylindrical propagation region in a direction in which the microwaves gather or are distributed. This is because the wave properties of the microwaves are dominant. FIG. 47 is the electric field strength distribution of the microwaves in the cylindrical propagation region, showing that the electric field strength distribution of the microwaves non-uniformly concentrates on the central portion in the cylindrical region. Therefore, the microwaves that are non-uniform in the cylindrical propagation region result in a non-uniform plasma, and a thin film is formed by this non-uniform plasma. This makes it difficult to form a uniform thin film. Moreover, as the diameter of the wafer is increased, the propagation region of the microwaves that generate plasma is increased, so that the concentration of the electric field strength distribution of the microwaves becomes significant, which makes it more difficult to generate a uniform plasma.
Furthermore, in the aforementioned method in which the non-uniform microwaves are uniformalized by a plasma absorption, it is difficult to maintain a balance between changes in the process conditions such as the flow rate and the composition ratio of gas, the pressure, and the sample temperature. Therefore, it is necessary to set the process conditions in accordance with the processing procedure, and difficult to maintain microwave uniformity. In addition, the process margins are small, that is, for example, even a small change in conditions caused by, for example, maintenance can change the process conditions.
In the method disclosed in JP 8-316198A, microwaves are introduced to a dielectric inside one processing chamber via a plurality of introduction windows, and therefore the microwaves introduced from each introduction window interfere with each other and become non-uniform. Such non-uniform microwaves generates a non-uniform plasma, which makes it difficult to perform uniform processing with respect to a sample surface because gas molecules are excited and activated by the non-uniform plasma.
In the method of uniformly introducing microwaves in a plasma apparatus to a dielectric with a slot plate, a thin slot plate or antenna that is formed with metal is used, and therefore heat generated during plasma generation is easily dissipated, so that the temperature in the plasma apparatus is increased and the slot plate is deformed by heat, resulting in distortion. Thus, a change in the characteristics such as a change in the transmission of the microwaves and deterioration of the uniformity of the microwaves will be caused by the slots.
Furthermore, there is an additional problem with respect to the recent demand for the miniaturization of ICs and an increase in the diameter of wafers. It is difficult to perform uniform processes with respect to a sample when performing processes such as film-formation, etching, ashing, sputtering, or cleaning in a conventional gas supply system. For example, when performing a film formation process or the like, a processing gas is introduced from a gas introduction portion to a processing chamber. At this point, microwaves enter the gas introduction portion from the processing chamber, and anomalous discharge or abnormal growth of films occurs in the gas introduction portion. Furthermore, since the gas introduction portion is formed by providing openings in the processing chamber, the microwaves in the processing chamber become non-uniform because of the non-uniform supply of gas, resulting in non-uniform plasma. This makes it difficult to perform uniform processing with respect to a sample.